Bentonite Bentonite is an absorbent swelling clay or powder consisting mostly of montmorillonite. It usually forms from weathering of volcanic ash in seawater, which converts the volcanic glass present in the ash to clay minerals. Types Sodium bentonite Calcium bentonite Potassium bentonite Uses Foundry Bentonite inflation index 20-30 Drilling Bentonite : API, OCMA, High Yield Bentonite for detergents Bentonite for ceramic